There are cases where a semiconductor integrated circuit includes multiple MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) having mutually-different gate threshold voltages. For example, such MOSFETs are provided on P-type or N-type wells having different carrier concentrations corresponding to the gate threshold voltages of the MOSFETs. However, to form the multiple wells having different carrier concentrations, it is necessary to perform ion implantation multiple times and use ion implantation masks having different patterns in the ion implantation processes. Therefore, the manufacturing efficiency of the semiconductor integrated circuit decreases; and the cost of the semiconductor integrated circuit increases.